1. Field of the Invention
The present invention relates to a method of electrostatically coating a non-conductive article. More specifically, the invention relates to a method and device for electrostatically painting plastic articles.
2. Description of the Related Art
Electrostatic painting has widely been used to paint conductive articles such as steel automobile bodies. Small droplets of paint are electrostatically charged and sprayed onto the metal surface. The metal surface is grounded and the charged paint droplets are attracted to the metal surface. Electrostatic painting reduces the amount of overspray and promotes the adhesion of the paint to the metal surface. Various methods of electrostatically painting non-conductive articles have been tried. Foremost among these methods is an attempt to electrically ground the article by attaching metal grounding wires to the plastic article to discharge the electric potential buildup on the surface of the article. It is difficult and time-consuming to ground plastic parts using metal wires.
The electrostatic painting process deposits a layer of electrostatically charged paint droplets atop a surface. If the surface is completely grounded, as in the case of metal articles, the electrostatic charge dissipates through the article into the ground. If the surface is non-conductive or improperly grounded, then areas of residual electrostatic charge remain atop the surface of the article. This residual electrostatic charge repels subsequent application of electrostatic paint having the same polarity. The resulting finish has a non-uniform paint thickness and poor surface appearance.
The problem of charge build-up was addressed in U.S. Pat. No. 5,222,663, issued to Noakes, et al., Jun. 29, 1993. Noakes, et al. teaches that electric charge tends to accumulate if an attempt is made to coat a plastic article with electrically charged particles. The accumulated charge tends to repel subsequent spray clouds of charged particles (see Column 4, Lines 33-39). Noakes et al. attempts to overcome this charge buildup by applying an alternating potential for electrostatically charging the paint. The paint separates into discrete ligaments, each having an opposing polarity. The frequency of the alternating electric potential is such that each cloud ligament of atomized particles is deposited onto the plastic's surface without discharging any particles from the preceding cloud. The spray clouds substantially neutralize each other on the target. Noakes et al. applies a pulsed stream of paint clouds. Physical separation of the oppositely charged paint clouds is necessary to avoid residual charge build-up.
The device taught by Noakes et al. is useful only with single sprayer applications. Multiple sprayers would cause interfering spray patterns whereby the electric charge of spray droplets would be discharged in the air above the target surface. Complex three dimensional parts often require two or more sprayers to completely coat the surface of the article. Additionally, small variations in the electronic signal applied to the sprayer causes fluctuations in the cloud ligament size and resulting paint finish. The complexities of the electronic components and the finish produced from a pulse spray discourage the use of this device.
Oppositely charged sprayers having the same electric potential have been used to aid in the atomization of liquid pesticides when sprayed from an airplane. U.S. Pat. No. 4,703,891, issued to Jackson et al., Nov. 3, 1987, teaches a device having two banks of sprayers. Each bank has multiple sprayers connected to an electric generator applying the same polarity to each sprayer within a bank. The banks apply electric fields having opposite polarities to prevent or reduce the static buildup on the sprayers. Excess electric potential is dissipated through the liquid droplets and each bank of sprayers maintains an approximately balanced electric charge.
The Jackson et al. device is intended to aid in the atomization of the liquid pesticide. Airplanes are necessarily ungrounded and susceptible to electric potential buildup on the spray heads. The electric potential buildup reduces the atomization of the liquid pesticide and, hence, the efficiency of the sprayers. To avoid charge buildup, the atomized mist forms an electrical circuit between the generators. Each bank of sprayers produces oppositely charged spray droplets. The droplets are intentionally intermingled to balance the electric potential of the mist. This balancing negates the resulting electric potential on each spray droplet and renders it unsuitable for use in a paint application process. Individual paint droplets would not have an electrostatic charge when they strike the surface of the article to be painted.
It is useful to provide a method and device for applying electrostatically charged paint droplets to a non-conductive surface whereby residual electrostatic potential on the painted surface does not repel subsequent paint applications. The device should permit multiple sprayers to apply a uniform layer of paint atop a complex three dimensional surface.